Riddles, brain puzzles and mathematical problems

[BAndrew]

Just to clarify for those who do not know anything about circuits: the word in the quesiton is across, not around.

Edit: Far from correct, BAndrew.

oops Didn't see across there.

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Just to clarify for those who do not know anything about circuits: the word in the question is across, not around.

So either both the three upper or both the two lower switches have to work? (or every switch of course)
Is that how it goes?

In that case I think it will be
(1/3 * 1/3 * 1/3) + (1/3 * 1/3) - (1/3 * 1/3 * 1/3 * 1/3 * 1/3) =
1/27 + 1/9 - 1/243 = 9/243 + 27/243 - 1/243 = 35/243 =~ 0.144 =~ 14,4%